[PATCH v4 5/5] dma: swiotlb: Remove SWIOTLB_ANY
From: Aneesh Kumar K.V (Arm)
Date: Mon Sep 14 2026 - 03:16:23 EST
swiotlb pools are currently allocated below the architecture's low
address limit by default. SWIOTLB_ANY overrides this and permits
allocation anywhere in directly mapped memory.
SWIOTLB_INIT_ADDRESSING_LIMIT now explicitly identifies callers which
need a pool that is reachable by devices with limited DMA addressing.
Use it as the placement constraint and remove SWIOTLB_ANY.
This changes the default behavior for callers which do not set either
flag. Previously, omitting SWIOTLB_ANY placed the default pool below the
architecture's low address limit and also constrained dynamically
allocated pools to the same range. After this change, omitting
SWIOTLB_INIT_ADDRESSING_LIMIT permits both static and dynamic pools to
use any directly mapped memory.
Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@xxxxxxxxxx>
---
Documentation/core-api/swiotlb.rst | 8 ++++----
arch/powerpc/mm/mem.c | 6 ++++--
arch/s390/mm/init.c | 2 +-
arch/x86/kernel/pci-dma.c | 2 +-
include/linux/swiotlb.h | 7 +++----
kernel/dma/swiotlb.c | 17 ++++++-----------
6 files changed, 19 insertions(+), 23 deletions(-)
diff --git a/Documentation/core-api/swiotlb.rst b/Documentation/core-api/swiotlb.rst
index 71b4e4c27eb5..06d9f88fdf7b 100644
--- a/Documentation/core-api/swiotlb.rst
+++ b/Documentation/core-api/swiotlb.rst
@@ -148,10 +148,10 @@ may also be adjusted
due to other conditions, such as running in a CoCo VM, as described above. If
CONFIG_SWIOTLB_DYNAMIC is enabled, additional pools may be allocated later in
the life of the system. Each pool must be a contiguous range of physical
-memory. The default pool is allocated below the 4 GiB physical address line so
-it works for devices that can only address 32-bits of physical memory (unless
-architecture-specific code provides the SWIOTLB_ANY flag). In a CoCo VM, the
-pool memory must be decrypted before swiotlb is used.
+memory. The default pool is allocated below the architecture's low address
+limit when it is needed for devices with limited DMA addressing. Otherwise,
+it may be allocated anywhere in directly mapped memory. In a CoCo VM, the pool
+memory must be decrypted before swiotlb is used.
Each pool is divided into "slots" of size IO_TLB_SIZE, which is 2 KiB with
current definitions. IO_TLB_SEGSIZE contiguous slots (128 slots) constitute
diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
index 006ed41097e9..1bdea6c03848 100644
--- a/arch/powerpc/mm/mem.c
+++ b/arch/powerpc/mm/mem.c
@@ -295,9 +295,11 @@ void __init arch_mm_preinit(void)
/*
* Since the guest memory is inaccessible to the host,
* devices always need to use the SWIOTLB buffer for DMA
- * even if dma_capable() says otherwise.
+ * even if dma_capable() says otherwise. The hypervisor has
+ * no addressing limitation, so the buffer may be allocated
+ * anywhere.
*/
- ppc_swiotlb_flags |= SWIOTLB_ANY;
+ ppc_swiotlb_flags &= ~SWIOTLB_INIT_ADDRESSING_LIMIT;
}
/*
diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
index 78f507e24ceb..ce10292447f1 100644
--- a/arch/s390/mm/init.c
+++ b/arch/s390/mm/init.c
@@ -166,7 +166,7 @@ static void __init pv_init(void)
virtio_set_mem_acc_cb(virtio_require_restricted_mem_acc);
/* make sure bounce buffers are shared */
- swiotlb_init(SWIOTLB_VERBOSE | SWIOTLB_ANY);
+ swiotlb_init(SWIOTLB_VERBOSE);
swiotlb_update_mem_attributes();
}
diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
index cd6dd151d634..a8070cae8253 100644
--- a/arch/x86/kernel/pci-dma.c
+++ b/arch/x86/kernel/pci-dma.c
@@ -89,7 +89,7 @@ static void __init pci_xen_swiotlb_init(void)
/* Xen can use a SWIOTLB pool anywhere in directly mapped memory. */
x86_swiotlb_flags &= ~(SWIOTLB_INIT_ADDRESSING_LIMIT |
SWIOTLB_INIT_FORCE_DISABLE);
- x86_swiotlb_flags |= SWIOTLB_INIT_REMAP | SWIOTLB_ANY;
+ x86_swiotlb_flags |= SWIOTLB_INIT_REMAP;
swiotlb_init_remap(x86_swiotlb_flags, xen_swiotlb_fixup);
dma_ops = &xen_swiotlb_dma_ops;
if (IS_ENABLED(CONFIG_PCI))
diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
index ef7a776ee508..77542f107b3f 100644
--- a/include/linux/swiotlb.h
+++ b/include/linux/swiotlb.h
@@ -15,13 +15,12 @@ struct page;
struct scatterlist;
#define SWIOTLB_VERBOSE (1 << 0) /* verbose initialization */
-#define SWIOTLB_ANY (1 << 1) /* allow any memory for the buffer */
/* Initialize a default-sized pool for devices with limited DMA addressing. */
-#define SWIOTLB_INIT_ADDRESSING_LIMIT (1 << 2)
+#define SWIOTLB_INIT_ADDRESSING_LIMIT (1 << 1)
/* Initialize a default-sized pool that requires architecture remapping. */
-#define SWIOTLB_INIT_REMAP (1 << 3)
+#define SWIOTLB_INIT_REMAP (1 << 2)
/* Do not initialize a pool unless SWIOTLB is explicitly required. */
-#define SWIOTLB_INIT_FORCE_DISABLE (1 << 4)
+#define SWIOTLB_INIT_FORCE_DISABLE (1 << 3)
/*
* Maximum allowable number of contiguous slabs to map,
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 6695da682f96..f8a77926b8b0 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -448,15 +448,10 @@ static void __init *swiotlb_memblock_alloc(unsigned long nslabs,
size_t bytes = PAGE_ALIGN(nslabs << IO_TLB_SHIFT);
void *tlb;
- /*
- * By default allocate the bounce buffer memory from low memory, but
- * allow to pick a location everywhere for hypervisors with guest
- * memory encryption.
- */
- if (flags & SWIOTLB_ANY)
- tlb = memblock_alloc(bytes, PAGE_SIZE);
- else
+ if (flags & SWIOTLB_INIT_ADDRESSING_LIMIT)
tlb = memblock_alloc_low(bytes, PAGE_SIZE);
+ else
+ tlb = memblock_alloc(bytes, PAGE_SIZE);
if (!tlb) {
pr_warn("%s: Failed to allocate %zu bytes tlb structure\n",
@@ -604,10 +599,10 @@ void __init swiotlb_init_remap(unsigned int flags,
#ifdef CONFIG_SWIOTLB_DYNAMIC
if (!remap)
io_tlb_default_mem.can_grow = true;
- if (flags & SWIOTLB_ANY)
- io_tlb_default_mem.phys_limit = virt_to_phys(high_memory - 1);
- else
+ if (flags & SWIOTLB_INIT_ADDRESSING_LIMIT)
io_tlb_default_mem.phys_limit = ARCH_LOW_ADDRESS_LIMIT;
+ else
+ io_tlb_default_mem.phys_limit = virt_to_phys(high_memory - 1);
#endif
/* if we have host or guest memory encryption */
--
2.43.0